clk: clk-xgene: Adjust six checks for null pointers

The script “checkpatch.pl” pointed information out like the
following.

	Comparison to NULL could be written ...

Thus fix the affected source code places.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>